In general, automatic flatness control for a rolling mill comprises roll cooling means including a plurality of coolant spray nozzles disposed side by side in a longitudinal direction of work rolls for controlling shapes or conditions of the work roll surfaces during rolling and a flatness meter for detecting broadwise flatness of rolled strip to generate an output signal to operate the roll cooling means. The coolant spray nozzles are independently controlled in accordance with the output signal from the flatness meter whereby thermal crown of the work rolls is adjusted so that the flatness of strip is properly controlled.
In one of the prior art references, in case a strip portion or portions are judged by the output signal from the flatness meter to have localized high tension in a longitudinal direction of the work rolls which corresponds to a broadwise direction of the strip, roll coolant stops being sprayed corresponding to the strip portion or portions while, in case another strip portion or other strip portions are judged to have low tension due to their elongation, roll coolant is sprayed onto the strip portion or portions. This is accomplished by on-off control of the coolant spray nozzles.
In another prior art reference, a flow amount of roll coolant is successively adjusted by coolant flow adjusting valves provided corresponding to the respective coolant spray nozzles so that the thermal crown is controlled.
However, in the prior art references, since the roll coolant is controlled in accordance with only the output signal from the flatness meter, it is made without any variation in control along the longitudinal direction of the work rolls, with the result that any control cannot be made so as to generate no localized buckle such as quarter buckle due to thermal crown. Furthermore, since there is no control logic to compensate for variation ratio of flatness in a direction in which flatness varies, there often occurs overshoot and, therefore, the flatness of the strip cannot be controlled with precision.